The present invention concerns a piezoelectric contamination detector for the determination of the mass or the film thickness of gaseous, liquid or solid substances, which are being adsorbed or condensed on a surface of a piezoelectric resonator.
Aboard launchers, satellites and other spacecraft, the contamination of critical parts due to outgassing and evaporation of the construction materials, or of coatings and lubricants, has to be limited by restrictive materials selection. In addition, the remaining contamination of these critical parts must be controlled in-flight.
Particularly endangered elements are optical elements (e.g., infrared optics, sensors, solar energy collectors), mechanical elements (e.g. valves) and structures with thermal control coatings and paints with defined thermo-optical properties (e.g., emissivity and reflectivity). The problem is constantly increasing since optics and sensors are more and more frequently cooled to cryogenic temperatures, and metallic materials more and more often replaced by organic components. Thus, continuous on-board control of contamination is necessary.
One of the most common applied measuring principles is based on the change of the resonant frequency of a piezoelectric resonator when additional mass is loaded through adsorbed or condensed substances onto the surface which is facing the contamination source.
Current detectors are equipped with two quartz crystals, one being the measuring crystal, the other being the reference crystal. The arrangement needs a configuration where one crystal is on top of the other thereby providing a covering against contamination of the reference crystal by means of the measuring crystal. The temperature of the measuring crystal should be controlled to any value within the working temperature range in order to enable selective determination of the different adsorbed or condensed substances. Temperature control can be achieved by means of a built-in thermoelectric module, by heat exchange through cooled or heated gases from outside, or by other convenient methods. It is mandatory for measuring techniques that the reference crystal be kept at the identical temperature since the resonant frequency is not only a function of the mass but also of the crystal temperature. The temperatures are most commonly measured in proximity to both crystals by means of a single discrete temperature sensing element.
Different authors have shown, however, that even with compact constructions considerable temperature differences between both crystals can be observed, producing uncontrolled, and hence not correctable, measurement errors. Furthermore it was observed, that it is not the real temperature of the measuring crystal which is measured with such a single discrete temperature sensor placed close to both crystals, but approximately the temperature of the crystal support. Finally, with such a detector arrangement, contamination of the reference crystal cannot completely be excluded.
In the described detector concept the reference crystal serves primarily as the temperature reference. Only an ideal detector wherein the temperature coefficients are zero would need only one single crystal. Since no such quartz crystal exists, a second quartz crystal, which is in the ideal case absolutely identical to the first, is used as a reference. However, unavoidable tolerances of fabrication of the crystal cut-angle result, in practice, in different temperature coefficients of both crystals thus introducing error into the measured value.
In the literature (Journal of Spacecraft, Vol. 17, No. 2, 1980, p. 153) a modified outgassing monitor was proposed by the application of a doublet-crystal to reduce the temperature difference between the measuring and reference crystals.
However, that solution has the disadvantage of crosstalking between the resonance zones due to coupling enabled between the different oscillation modes of the measuring and reference areas. Such coupling is the origin of undesired frequency instabilities and interference phenomenas.
To permit re-evaporation of the adsorbed and condensed substances, the measuring crystal needs to be baked out. For this reason, the aforementioned concepts use an additional discrete heating element. In some cases the same aforementioned discrete, resistive temperature sensor also can be used for this goal. Since the heat transfer is in this case primarily indirect by means of radiation dissipation, the thermal efficiency is poor and the electrical power consumption is therefore important.